1. Field of the Invention
The present invention relates generally to electron beam (e-beam) apparatus and e-beam inspection methods.
2. Description of the Background Art
Most conventional wafer and mask inspection systems use light optical images. Scanning electron beam microscopes (SEMs) have also been developed for inspection and critical dimension (CD) measurement. These SEM instruments scan a very small beam over the surface, and record the re-emitted secondary electrons in a single detector. Image acquisition tends to be slower for SEMs than for direct imaging light optical instruments because only one image element (pixel) at a time is recorded.
A low energy emission microscope (LEEM) is a direct imaging (as opposed to scanning) electron microscope. A conventional LEEM uses a single illumination beam which is accelerated typically to about 10 to 30 keV (kilo electron-volts) in an electron gun. The single beam passes through a separator magnet that bends the beam into the axis of the objective lens. An image of the gun crossover is transferred to the back focal (diffraction) plane of the objective lens, forming a parallel flood beam that uniformly illuminates the substrate. The substrate is electrically floated at approximately the same voltage as the cathode of the electron gun, so that illuminating electrons are decelerated in the objective lens, striking the substrate at energies typically between 0 to about 1,000 eV (electron-volts). Some examples of prior art LEEM systems are described in the review paper: “The continuing development of the low energy electron microscope for characterizing surfaces,” L. Veneklasen, Rev. Sci. Inst. 63(12) p. 5513 (December 1992) and its references.
Insulating surfaces are generally not a problem for light optical inspection because the scattering and reflection of light is insensitive to electrostatic surface charge. Unfortunately, surface charging effects can pose a difficulty for electron beam imaging of insulating surfaces (whether scanned or direct imaging). The rate that a given pixel element charges depends upon the difference between electron flux arriving at and leaving each pixel. The high current densities required for imaging at inspection rates imply a likely high rate of charging if the electron flux leaving the surface is not balanced by that entering. Thus, the surface voltage can quickly reach levels detrimental to imaging or even, in some instances, detrimental to sample integrity. Effective means for controlling local surface charging are therefore desirable if e-beam instruments are to be used for inspection of wafers, masks and other non-conductive substrates.